i actually did something exactly like this and ran it for a few
months. i had scored a 4x400 mhz enterprise 450 from my job for fairly
cheap and wanted to put such an impressive beast of a machine to use
on my network, but didn't have as many sca drives in spud sleds as i
would have liked. so, i did this -- i ripped out the sca drive carrier
chassis -- it can be done fairly easily. get rid of that sca backplane
board, too. you can take that ata-power-supply like cable and splice
real drive power plugs off of it. just use a multimeter to probe
around to find out what is 5v, 12v, gnd. then, trace back the scsi
cable through the little passage over to the logic side of the
chassis. i am a scsi lover so what i did personally is rip out the sun
supplied one position 68-pin cable and threaded through a pair of nice
long scsi cables in place of it -- 68 pin and 50 pin -- with a bunch
of connectors on them, so lots of drive potential. pull out the main
board and you can see fairly easily how the scsi cables thread through
it. then i just plopped some regular old 68-pin scsi drives in there
and sat them on rubber feet on the bottom of the enclosure on the
drive-side. if i had been feeling more motivated (and had more
drives), i might have built a little frame to sit in there and hold
drives -- would have been pretty easy to do. it worked great for a few
months, running netbsd/sparc64 in 32-bit uniprocessor mode.
unfortunately it suffered some kind of catastrophic main board failure
and crapped out completely.. i had to replace it with an ultra 60 in a
pinch. pity, it looked sweet as hell. i was really disappointed that
it wasn't able to hold out until netbsd/sparc64 supported
multiprocessing.

openfirmware blows any pc bios out of the water as far as im concerned
but it will probably not like to boot from any sort of drive
controller from pc-land because it will be lacking the necessary
firmware to make it come up. i've never tried sata on sparc64 but i
would assume that if there is hardware-ind. support for your card it
might work if you bootstrapped the system from a scsi drive

all in all these are fun systems and they look really cool, but if it
happens to go belly up, its difficult to find spares and you pay a
premium over boxes e.g. ultra60/80 that are more common and offer
generally similar performance (if you don't need 14 pci slots, lol)

have fun, -S